Fixed panic when header is verifying for (Verifying | Stored) block

[svyatonik]

closes #97

This has been introduced recently, when headers verification has been moved to the separate thread. In some (rare) cases, we may have end up in situation when (at the same time) block' header is verifying in light_verification_thread and the block itself is verified in the heavy_verification_thread. Once both verifications are completed, block is stored in DB, but it is marked as Scheduled by the sync.

[tearodactyl]

Great to see that you found an interaction between threads, those are notoriously hard to catch.
May I suggest that you start tagging your 'master' to introduce some baselines, even if you are not ready to bake a 0.0.1 release.
That way interested parties, like myself will be able to pull in commits, re-test and report the outcomes in an organized fashion.